The present invention provides an oxygen storage material containing cerium oxide as well as a process for its preparation and its use for the catalytic conversion of substances, in particular for the treatment of exhaust gases from internal combustion engines.
Internal combustion engines emit as primary pollutants, with the exhaust gases, carbon monoxide CO, unburnt hydrocarbons HC and nitrogen oxides NOx, a high percentage of which are converted by modem exhaust gas treatment catalysts into the harmless components water, carbon dioxide and nitrogen. Conversion takes place substantially under stoichiometric conditions, that is to say the oxygen present in the exhaust gas is controlled with the aid of a so-called lambda probe in such a way that the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides to nitrogen can take place almost quantitatively. The catalysts developed for this purpose are called three-way catalysts. They usually contain, as catalytically active components, one or more metals from the platinum group in the Periodic Table of Elements on high surface area support materials such as γ-aluminum oxide with specific surface areas of more than 50 m2/g.
Stoichiometric conditions are present when the air/fuel ratio λ is 1. The air/fuel ratio λ is the air/fuel ratio normalized to stoichiometric conditions. The air/fuel ratio states how many kilograms of air are required for complete combustion of one kilogram of fuel. In the case of conventional gasoline engine fuels, the stoichiometric air/fuel ratio has a value of 14.6. Substoichiometric exhaust gas compositions with λ<1 are called rich and superstoichiometric compositions with λ>1 are called lean.
The engine exhaust gas, depending on the load and the speed of revolution of the engine, experiences relatively large periodic variations in air/fuel ratio around the value 1. For better conversion of oxidizable pollutant components under these dynamic conditions, oxygen storage components such as, for example, cerium oxide, are used which bind oxygen by changing the oxidation state from Ce3+ to Ce4+ when it is present in excess and release it again for the oxidative reaction by changing from Ce4+ to Ce3+ when there is a insufficiency of oxygen in the exhaust gas.
Car exhaust gas catalysts are subjected to exhaust gas temperatures of up to 1100° C. These high temperatures require the use of appropriate materials for the catalysts which are thermally resistant and have long-term stability.
EP 0 207 857 B1 describes a material based on cerium oxide which contains as essential components cerium oxide and an additive consisting of at least one of the oxides of the metals A from the group aluminum, silicon, zirconium and thorium. The material, with a specific surface area of more than 10 m2/g, is stable up to a firing temperature of 900° C.
EP 0 444 470 A1 describes a high surface area cerium oxide which consists of an intimate mixture of cerium oxide with 5 to 25 mol %, with respect to moles of cerium oxide, of a cerium oxide stabilizer. Lanthanum, neodymium and yttrium are mentioned as stabilizers. The material is obtained by co-precipitation from a common solution of a cerium oxide precursor and a precursor of the cerium oxide stabilizer and subsequent calcination in air at temperatures above 500° C. The BET surface area of this material after calcination at 980° C. for a period of 4 hours is still more than 20 m2/g.
EP 0 337 809 A2 describes a catalyst composition which contains, inter alia, zirconium oxide particles stabilized with cerium oxide. The zirconium oxide particles are stabilized with cerium oxide by impregnating zirconium oxide with a cerium salt solution. The impregnated particles obtained therefrom are dried and calcined until the graphical representation of the X-ray diffraction spectrum no longer shows a peak for the crystalline form of cerium oxide. Cerium oxide is present in the cerium oxide/zirconium oxide mixture in an amount of 10 to 50 wt. %, with respect to the zirconium oxide. In addition to the cerium salt, an yttrium and/or calcium salt may also be used. The X-ray diffraction spectrum, after the material has been calcined for 10 hours in air at a temperature of 900° C., shows only a peak for tetragonal zirconium oxide and no peak for cerium oxide. Cerium oxide is thus present in this material substantially in the form of a solid solution with the zirconium oxide.
EP 0 827 775 A1 describes an oxygen storage mixed oxide. The mixed oxide consists of cerium oxide or a cerium/zirconium mixed oxide which is loaded with praseodymium oxide. The molar ratio of praseodymium to cerium in the mixed oxide is between 1:4 and 4:1.
WO 98/42437 describes a catalyst composition which contains a mixed oxide of cerium and praseodymium and optionally one or more other rare earth oxides. The atomic ratio Pr:Ce is in the range between 2:100 and 100:100. The mixed oxide may be obtained by co-precipitation or by impregnating cerium oxide particles with a praseodymium precursor compound and then calcining in air and has, compared with pure cerium oxide, an improved oxygen storage capacity.
WO 98/45027 describes another catalyst composition which contains an oxygen storage material with improved oxygen storage capacity. The oxygen storage material is a mixed oxide which contains oxides of cerium, neodymium and zirconium. The material can be obtained by co-precipitation of compounds of zirconium and the rare earth metals followed by calcination in air.
The known oxygen storage materials are used in three-way catalysts for the treatment of exhaust gases from stoichiometrically operated internal combustion engines. An essential criterion for assessing these materials is thus their ability to improve the conversion of carbon monoxide and nitrogen oxides under the dynamic conditions prevailing in the exhaust gas. One measure of the dynamic conversion is the point of intersection of the conversion curves for carbon monoxide and nitrogen oxides, the so called “cross-over” point for exhaust gas compositions varying periodically between rich and lean. The conversion measured at the cross-over point is the highest conversion which can simultaneously be obtained for carbon monoxide and nitrogen oxides and is a measure of the rate at which it is possible to change the oxygen storage material being used from one oxidation state to the other. This property of the storage material is called the dynamics in the following. The higher the conversion at the cross-over point, the better also is the dynamic behaviour of the catalyst under consideration.
An object of the present invention is to provide an oxygen storage material which has better dynamic behaviour than the materials described hitherto. Further objects of this invention are a process for preparing this material and the use of this material in exhaust gas treatment catalysts for internal combustion engines.